1. Field of the Invention PA1 2. The Prior Art
This invention relates to scanning probe microscopes, in particular the scanning tunneling microscope (STM) and the atomic force microscope (AFM). Identification of the chemical composition of areas in a microscope image has been very difficult in the past. Certain elements can be identified by their characteristic x-ray emission in an electron microscope. In the case of the STM, identification has been limited to certain atoms which induce well-understood surface states near the Fermi energy (the energy of the tunneling electrons). In the case of the AFM, certain discrimination between the composition of molecular adlayers has been possible, based on differences in friction between the adlayers and the scanning tip. In the present invention, we exploit the ability of a conductive tip to transfer charge to and from molecules in surfaces at well defined potentials (being the electrochemical reduction or oxidation potentials). The present invention permits discrimination among, and identification of electroactive molecules on the surface of a sample. The present invention provides the first known method for identification of organic molecules in a microscope with nanometer scale resolution.
The scanning tunneling microscope (STM) is capable of atomic-resolution imaging of a conductive surface [Binnig, G. and Rohrer, H., Reviews of Modern Physics, vol. 59, pp. 615-626, 1987]. The atomic force microscope can image single atoms in an insulating surface [Ohnesorge, F. and Binnig, G., Science vol. 260, pp. 1451-1456, 1993]. However, neither technique is well suited to identification of the composition of material in the gap formed between the probe and an underlying substrate. In the case of the STM, current is carried by electrons in the itinerant states of the metals that constitute the tip or substrate. The composition of some intervening material is only of significance to the extent that it modifies the properties of those states. In certain very special cases, it has proved possible to identify surface atoms, based on the manner in which they modify the current carrying states near a surface [Feenstra et al., Physical Review Letters, vol. 58, pp. 1192-1195, 1987]. In the case of the AFM, the intervening material plays a role in the friction between the scanning probe when there are chemically-specific interactions between the scanning probe and molecules under the probe, a phenomenon that has been used to distinguish (but not identify) regions of different chemical composition in a thin film [Overney et al., Nature, 359, pp. 133-135, 1992].
An alternative approach to chemical identification uses thin films sandwiched between metal electrodes. A voltage is applied between the electrodes so as to raise the energy of the electrons in one electrode with respect to the other electrode. When the energy of electrons in one electrode is coincident with an electronic state of a molecule in the thin film between the electrode, an enhanced current flow occurs because of the process of resonant tunneling, a quantum-mechanical phenomenon in which the intermediate state in the gap serves to transport extra current. Because the energy of the molecular state is characteristic of the chemical species in the gap between the electrodes, the voltage at which this extra current flows is characteristic and could, in principle, be used to identify the chemical species. FIG. 1 shows a schematic arrangement of such a solid-state tunnel junction 10. A voltage V is applied by device 12 across two metal electrodes, 14 and 16. Each electrode is coated with a thin insulating film (such as an oxide layer) 18 and 20 and a layer of molecules 22. A sensitive current measuring device 24 records the current through the device. FIGS. 2A and 2B show the energy of the electrons in the electrodes of FIG. 1 schematically in two configurations: FIG. 2A is a diagram of voltage conditions where there is no extra current due to resonant tunneling. The voltage applied across the device, V.sub.1 is too little to raise the energy of the electrons in electrode 14 so as to be coincident with the energy of the molecular state E.sub.M. FIG. 2B is a diagram of the situation when the voltage is adjusted to resonance. The electrons that carry current from electrode 14 now have an energy equal to E.sub.M. The voltage, V.sub.2 at which the extra current "turns on" serves to identify the molecule in the gap. A diagram of the current-voltage characteristic of such a device is shown in FIG. 3. Conventionally, the step at V.sub.2 is detected by plotting the first or second derivative of the current so that features are made sharper.
While the above description has long been supposed to apply to tunneling through molecules, some recent work shows that the situation is both more complex, and yet more tractable in terms of achieving the desirable goal of identifying a broad range of molecules by such a mechanism. Mazur and Hipps [Journal of Physical Chemistry, submitted, 1994] have measured the current-voltage characteristics of a number of devices containing different organic molecules with states that lie some electron volts from the energy of the electrons with no voltage applied across the device. They have extracted the value of the voltage at which the extra current turns on (V.sub.2 in FIG. 3) for a number of different organic molecules. They find that the energy of the state, E.sub.M, at which increased current flow is detected, is not the energy that would be measured for the same molecule in the gas phase. It is, instead, the energy of the final state that occurs when the molecule is electrochemically reduced or oxidized. This is different from the energy of the isolated molecule for two reasons. First, oxidation or reduction involves charging of the molecule, a process that changes the energy of the states of the molecule. In contrast, in resonant tunneling, the process described above, the electron does not interact with the molecule for long enough to change its energy. Second, the charged molecule is embedded in a dielectric medium. In this case it is the insulating films 18, 20 and other molecules that constitute the dielectric, but in an electrochemistry experiment, it is the solvent used to dissolve the molecules. In either case, the medium polarizes so as to reduce the energy of the charged state. This final step is called `relaxation`. In any case, the charging (reduction) or discharging (oxidation) of a molecule in a medium is a much more complex process than resonant tunneling. However, the magnitude of the energy shift caused by relaxation is usually big; i.e., many electron volts, so that the states associated with oxidized or reduced molecules lie closer to the energy of the electrons in the metal than the original, unperturbed, states of the molecule. More importantly, from the standpoint of the present invention, these state-energies are easily measured by the conventional methods of electrochemistry. Many sources list standard reduction and oxidation potentials for organic compounds.
Electrochemical potentials are conventionally stated as potentials relative to the electrochemical potential of a standard `reference electrode`. Thus, identification of molecules via their reduction or oxidation potentials would seem to require an electrochemical cell containing such an electrode as a reference. However, these reference electrodes function because their potential is fixed. That is to say, a certain fixed amount of work would have to be done to remove an electron from such an electrode to a position at rest far from the electrode. This quantity is the work function of the reference electrode. It is illustrated schematically in FIG. 4 where the energy to take an electron from the reference electrode is labeled .phi..sub.REF. The (known) oxidation and reduction potentials are labeled E.sub.RER (OX) and E.sub.RER (RED). The work function of the metal used for an electrode in a tunneling device is also usually a known quantity, .phi..sub.METAL. Thus, the voltage for reduction (V.sub.2 (RED)) or oxidation (V.sub.2 (OX)) of molecules in a device like that shown in FIG. 1 can be calculated if .phi..sub.REF is known. The standard for reference electrodes is the Normal Hydrogen Electrode, NHE, and values for the potential of other reference electrodes relative to the NHE are well known. The work function of the NHE, .phi..sub.NHE, is still the subject of some debate, although 4.8 eV is the currently accepted value [Trasatti, S., Advances in Electrochemistry, ed. H. Gerischer, C. W. Tobias, Wiley InterScience, New York, pp. 213-321 ].
Mazur and Hipps have used the value of .phi..sub.NHE =4.8 eV together with the known oxidation and reduction potentials of several organic molecules and the work function of lead (4.1 eV) to calculate the reduction potential for these molecules between lead electrodes in a device such as that shown in FIG. 1. The potential is calculated as V.sub.2 (RED), the voltage that would have to be applied to the device in order to see a step-like increase in current due to the reduction of the molecules. TABLE 1 includes a listing of the calculated V.sub.2 (RED), and the measured voltage, V.sub.2, at which a step occurs in the current for six organic molecules.
TABLE I ______________________________________ V.sub.2 V.sub.2 Molecule (RED) Calculated (Volts) Measured (Volts) ______________________________________ Ni(acac).sub.2 2.47 2.50 coronene 2.62 2.51 anthracene 2.80 2.74 perylene 3.00 2.91 tetracene 3.12 3.11 pentacene 3.40 3.47 ______________________________________
On the whole, there is rather good agreement between the calculated and measured values. Thus, this step in current at V.sub.2 serves as a marker that may be used to identify the organic compound. Clearly, this method can be extended to other organic compounds and other electrodes. Compounds that are reduced at more negative potentials could be studied on electrodes with larger work functions. Thus, this method of chemical identification is applicable to any compound that can be reduced (or oxidized) on any metal suitable for use as an electrode.
The limitation of the prior art is that, in order to carry out identification of molecules, they must be somehow inserted into a device of the general layout shown in FIG. 1. This is not easy to do and not very useful once done, for one must usually know the chemistry of the molecules in advance in order to make a device such as that shown in FIG. 1.